The present invention relates to a motor vehicle with a line pressure control for an automatic transmission, and more particularly to a line pressure control during occurrence of a shifting between one gear ratio and another gear ratio in an automatic transmission.
In the automatic transmission, a desired gear ratio is established by hydraulically activating selected one or ones of various friction devices (such as clutches and brakes) by line pressure, and a shift between two gear ratios is effected by changing friction device or devices to be activated.
If the line pressure is excessively high, transient engagement capacity of the friction device becomes excessively high, causing great shift shock to occur, whereas if the line pressure is excessively low, the transient engagement capacity of the friction device becomes excessively low, causing a slip to occur, thus shortening an operating life of the friction device. Thus, the line pressure has to be appropriately controlled. Conventionally, as described in the publication entitled "SERVICE MANUAL FOR AUTOMATIC TRANSMISSION OF THE RE4R01A TYPE" (A261C07) published in March 1987 by N1SSAN MOTOR COMPANY, LIMITED, a drain circuit of a line pressure regulator valve is opened or closed by a line pressure solenoid of the duty cycle type. The duty varies from 0% to 100%. When the duty is 0%, the line pressure solenoid is left OFF, closing the drain circuit, while when the duty is 100%, the line pressure solenoid is left ON, opening the drain circuit. Thus, increasing the duty causes the line pressure regulator to increase a magnitude of line pressure generated thereby. Various values of the duty are contained in a table data stored in a ROM of a microcomputer of an automatic transmission control unit. Different table data are provided, one for use during the shifting operation, and another for use at the normal stable non-shifting operation, for example. The duty values are retrievable out of each table data by a table look-up operation using a variable such as a throttle opening degree.
However, this conventional line pressure control cannot cope with a situation where the line pressure solenoid has a manufacturing variation and/or a degradation of characteristic over time, or a situation where the friction device has a manufacturing variation and/or a wear degradation over time. In the former situation, even if the line pressure solenoid is subject to the same duty, the magnitude of line pressure deviates from a target value. In the latter situation, even if the magnitude of line pressure is adjusted to the target value, the friction device does not show a desired performance characteristic. Thus, in any event, the conventional line pressure control fails to avoid occurrence of substantial shift shock or reduction in operating life of the friction device.
When a shift solenoid is switched from ON state to OFF state at a time in a manner to effect a 1--2 upshift of the automatic transmission, a pressure level for the second gear operation increases so as to induce the engagement of a given friction device. This produces a change in a rotational speed ratio of a transmission input shaft to a transmission output shaft from a value corresponding to the first gear to that which corresponds to the second gear. A process of the change in the rotational speed ratio at a low line pressure level is different from that at a high line pressure level. Accordingly, it is possible to determine from a time required for the change in the rotational speed ratio, viz., the duration of an inertia phase, whether an actual line pressure is adequate or not. Thus, a line pressure control is proposed in the co-pending U.S. Patent Application Ser. No. 07/289,050, now U.S. Pat. No. 4,981,053, in which the duration of the inertia phase is measured during shifting, and the measured duration is compared with a target value. A correction value, which is obtained from the difference between the two, is used for adjusting the line pressure during the next shifting. Such a control is called a learning control.
The automatic transmission is controlled in response to signals from various sensors which indicate operating conditions of a motor vehicle such as an engine operating state. Thus, the signals are transmitted through a communication line from an engine control unit to an automatic transmission control unit. However, if the line pressure learning control is kept employed even though it cannot be judged whether an engine system operates normally or not, due to an abnormality of the communication line, the line pressure learning control may actually degrade the quality of shifting by using the inaccurate learning data obtained.
The present invention aims at improving a line pressure control in a motor vehicle such that line pressure learning control is not employed when a communication line between an engine control unit and a transmission control unit is in an abnormal state.